When examining fallen storm trees that have split cleanly down the center, certified arborists almost always identify a common anatomical defect: codominant stems with included bark. This structural flaw acts like an invisible wedge inside the trunk, turning normal wind dynamics into catastrophic mechanical failure. Understanding the difference between strong branch unions and dangerous V-crotches is essential for identifying hazardous trees on your property before severe weather hits.
1. The Biomechanics of "Included Bark"
In a healthy tree, a single dominant central trunk produces smaller lateral branches. Wood fibers from the trunk grow around the branch base, creating a powerful interlocking collar capable of supporting massive dynamic weight.
When two or more main stems grow upward at equal diameter from a shared crotch (codominant stems), a severe biomechanical conflict arises:
- As both trunks expand in girth each growing season, they press tightly against each other inside the narrow crotch.
- Instead of wood fibers connecting, tree bark becomes trapped and pinched between the stems—a condition known as included bark.
- Because bark consists of dead, corky cells with zero structural tensile strength, there is no physical wood connection between the two trunks above the crotch seam.
2. How Wind Forces Exploit the V-Crotch Wedge
During a thunderstorm or high-wind squall, wind pushes the two separate canopies in opposing directions. Because the stems are not fused together:
- The included bark acts like a mechanical wedge driven between two wooden blocks.
- Dynamic leverage multiplies at the base of the crotch seam.
- Once wind gusts exceed 45 to 55 mph, the seam tears apart, causing one or both massive multi-ton trunks to collapse onto adjacent roofs or power lines.
3. Arboricultural Solutions: Cabling, Bracing & Pruning
Crown Reduction Pruning
Certified arborists prune the outer foliage of the weaker codominant stem, reducing wind resistance and dynamic sail weight so the union experiences less lateral torque during storms.
Dynamic Synthetic Cabling (Cobra Systems)
For high-value shade trees, arborists install dynamic, flexible synthetic cables (such as Cobra or Boa systems) between the two trunks in the upper two-thirds of the canopy. The cables allow gentle movement during calm weather but arrest violent sway during severe storms, preventing the crotch from tearing open.
Rigid Steel Rod Bracing
If the crotch has already begun to hairline-crack, threaded steel rods are drilled through the union with heavy washers to physically bolt the two stems together in conjunction with overhead cables.
Complete Hazardous Removal
If decay or fungal conks have developed inside the cracked seam, cabling is no longer safe. The tree must be safely extracted by crane before the next gale-force storm.
Walk around your property and look up into the main crotches of your mature oaks, maples, and ashes. If you spot a tight V-shaped seam with a sunken vertical crack or oozing moisture, call an ISA-Certified Arborist for an immediate structural evaluation.